The invention relates to a source of vapour-depositing manganese from the solid state onto a substrate in a vacuum. This vapour deposition source is used in particular for forming a MnO-layer by oxidation on the substrate for a photosensitive layer or for a secondary electron-emitting layer of a photo-electric tube. The manganese oxide layer serves as a substrate for the photosensitive layer or for the secondary electron-emitting layer. The invention also relates to a method of manufacturing such a vapour deposition source.
Such sources for vapour-depositing manganese are known from U.S. Pat. No. 3,637,421. In these sources a binary mixture or a binary alloy of manganese and other metal, for example nickel, is used in such manner that the melting-point of the mixture or alloy in the same vacuum conditions is lower than that of manganese alone. The other metal is chosen such that its vapour pressure is very low and is at least 100 times smaller than that of manganese so that during vapour-deposition only insignificant traces of the other metal are present in the layer. According to the Patent Specification the vapour deposition source is a molten manganese mixture or alloy provided in a melting crucible made of a material such as tantalum which reacts only slightly with the mixture or alloy. Said melting crucible is heated, for example, by electron bombardment. In general such a source forms part of a bulky vapour deposition plant and cannot be used in a photo-electric tube during the manufacture thereof to obtain an intermediate layer of MnO. In tube manufacture it is preferable to use a vapour deposition source symmetrically disposed opposite to the surface of the substrate to be coated, which can be permanently provided in the tube, the vapour deposition taking place by sublimation (the transition of the material from the solid state into the vapour state). Such sources are known in the form of manganese beads which are provided at a distance from each other on an electrically conductive wire which consists, for example, of molybdenum or tungsten. This source is provided, for example, in a photoelectric tube in the form of a wreath which is situated opposite to the inner surface of the window of the tube which serves as a substrate for the photo-sensitive layer. The manganese is heated and evaporated by means of a current flowing in the wire conductor. The formation and the connection of the beads to the wire conductor are carried out by fusing small quantities of manganese in the form of powder or small particles in contact with the wire and then cooling again. It is necessary for the wire to be heated to a temperature of at least 1250.degree. C. because a matrix of refractory oxide (MnO) surrounds the manganese. On the one hand, only beads of small dimensions are thus formed so that the quantity of evaporable manganese is limited. On the other hand the molybdenum or tungsten, after having been heated at these temperatures, becomes brittle upon cooling so that the vapour deposition source formed by the chain of beads thus obtained is fragile and difficult to handle.